The Federal Communication Commission (FCC) has requested all cellular and Personal Communication System (PCS) based operators to provide emergency 911 location services for mobile-telephones. Solutions to this challenging issue include typical prior art systems that use well-known location techniques, such as time difference of arrival (TDOA), ranging, angle-of-arrival, etc., to yield a good estimate of a mobile-telephone's location. These location techniques typically involve the use of times-of-arrivals observed at a single location or distributed locations together with "triangulation" principles to determine a mobile-telephone's location.
To determine a mobile-telephone's location to an acceptable accuracy using triangulation principles, one detector should be able to detect signals transmitted from at least three signal sources and record the times-of-arrivals of each signal at the detector. FIG. 1 illustrates a wireless communication system 1 adapted to use forward link signals, i.e., signals transmitted from base stations to a mobile-telephone, for locating a mobile-telephone 6 within the coverage area of the wireless communication system 1. The wireless communication system includes a plurality of base stations 4-1, . . . , 3. To locate a mobile-telephone 6 within the coverage area of the wireless communication system 1, the mobile-telephone 6 should be able to detect signals from at least three surrounding base stations and record times-of-arrivals for each of the detected signals. Using the recorded times-of-arrivals and the known locations of the corresponding base stations, the mobile-telephone's location can be estimated using triangulation principles.
The above described wireless communication system can also utilize reverse link signals, i.e., signals transmitted from the mobile-telephone to the base stations, to estimate the mobile-telephone's location. When using reverse link signals for location purposes, at least three base stations should be able to detect a signal transmitted from the mobile-telephone and record the times-of-arrivals at each of the base stations which are then used to estimate the mobile-telephone's location.
Regardless of the signal source, the accuracy of a triangulation based estimate of a mobile-telephone's location depends, in part, on the ability of one or more detectors to detect line-of-sight signals. Thus, if a detector does not detect the line-of-sight signals, it cannot record times-of-arrivals for the line-of-sight signals.
Generally, a signal may travel more than one path from the signal source to the detector. The signal can either travel a direct path or a reflected path to the detector. The signal traveling the direct path is the line-of-sight signal, whereas the signal traveling the reflected path is a multi-path or a non-line-of-sight signal. Detectors assume that the first signal it receives is the line-of-sight signal because such signal travels a more direct path (than non-line-of-sight signals) and, thus, should arrive at the detector before any non-line-of-sight signal. However, if the detector is unable to receive or hear the line-of-sight signal, it may assume a first arriving non-line-of-sight signal to be a line-of-sight signal. Since non-line-of-sight signals travel a greater distance than line-of-sight signals, the times-of-arrivals for non-line-of-sight signals will be later than those for line-of-sight signals. Accordingly, if times-of-arrivals for non-line-of-sight signals are used to estimate a mobile-telephone's location, then the accuracy of the estimate is adversely affected.
Whether a detector receives a line-of-sight signal depends mostly on the amount of attenuation or degradation the signal undergoes as it travels from the signal source to the detector and the power level at which the signal was transmitted. Generally, all signals attenuate or degrade as they travel from the signal source to the detector. The amount of attenuation or degradation depends on the environment through which it travels--for example, in urban environments where more obstacles, e.g., buildings, are likely to be between the signal source and the detector, the amount of attenuation or degradation to line-of-sight signals will generally be greater.
To compensate for possible attenuation or degradation of the line-of-sight signals, the wireless communication systems (utilizing reverse link signaling for location purposes) may require the mobile-telephone to transmit its signal at a higher power level. This would increase the signal-to-noise ratio at the base station, thereby offsetting the attenuation (and/or degradation) while increasing the likelihood of detecting the line-of-sight signals. However, increasing the power level of the reverse link signals significantly raises the interference level, i.e., noise, for other mobile-telephones in the same or neighboring cells.
One possible solution for increasing the signal-to-noise ratio of the line-of-sight signal without prolonged interference level increases involves the transmission of reverse link signals at higher power levels for short time periods. This solution, however, is difficult to implement without changing the mobile side of existing wireless communication standards. Specifically, the existing wireless communication standards provide guidelines for dynamically controlling the power level of reverse link signals based on strict factors such as distance from the mobile-telephone to the serving base station. The introduction of a new factor would require the development of a new dynamic reverse link power control scheme that takes into consideration this new factor, i.e., increasing reverse link signal power levels for short durations, thereby requiring a change to the mobile side of the wireless communication standards. Implementation of such changes (to the mobile side) would require corresponding upgrades to all mobile-telephones in the wireless communication network, thereby making the aforementioned solution difficult to implement. Accordingly, there exists a need for a method for accurately estimating a location of a mobile-telephone without significant increases to interference levels or changes to the mobile side of existing wireless communication standards.